Many systems require a wiring and/or electrical contacts in order to supply electrical power to devices. Omitting these wires and contacts provides for an improved user experience. Traditionally, this has been achieved using batteries located in the devices but this approach has a number of disadvantages including extra weight, bulk and the need to frequently replace or recharge the batteries. Recently, the approach of using wireless inductive power transfer has received increasing interest.
Part of this increased interest is due to the number and variety of portable and mobile devices having exploded in the last decade. For example, the use of mobile phones, tablets, media players etc. has become ubiquitous. Such devices are generally powered by internal batteries and the typical use scenario often requires recharging of batteries or direct wired powering of the device from an external power supply.
As mentioned, most present day devices require a wiring and/or explicit electrical contacts to be powered from an external power supply. However, this tends to be impractical and requires the user to physically insert connectors or otherwise establish a physical electrical contact. It also tends to be inconvenient to the user by introducing lengths of wire. Typically, power requirements also differ significantly, and currently most devices are provided with their own dedicated power supply resulting in a typical user having a large number of different power supplies with each power supply being dedicated to a specific device. Although, internal batteries may prevent the need for a wired connection to an external power supply, this approach only provides a partial solution as the batteries will need recharging (or replacing which is expensive). The use of batteries may also add substantially to the weight and potentially cost and size of the devices.
In order to provide a significantly improved user experience, it has been proposed to use a wireless power supply wherein power is inductively transferred from a transmitter coil in a power transmitter device to a receiver coil in the individual devices.
Power transmission via magnetic induction is a well-known concept, mostly applied in transformers which have a tight coupling between the primary transmitter coil and the secondary receiver coil. By separating the primary transmitter coil and the secondary receiver coil between two devices, wireless power transfer between the devices becomes possible based on the principle of a loosely coupled transformer.
Such an arrangement allows a wireless power transfer to the device without requiring any wires or physical electrical connections. Indeed, it may simply allow a device to be placed adjacent to, or on top of, the transmitter coil in order to be recharged or powered externally. For example, power transmitter devices may be arranged with a horizontal surface on which a device can simply be placed in order to be powered.
Furthermore, such wireless power transfer arrangements may advantageously be designed such that the power transmitter device can be used with a range of power receiver devices. In particular, a wireless power transfer standard known as the Qi standard has been defined and is currently being developed further. This standard allows power transmitter devices that meet the Qi standard to be used with power receiver devices that also meet the Qi standard without these having to be from the same manufacturer or having to be dedicated to each other. The Qi standard further includes some functionality for allowing the operation to be adapted to the specific power receiver device (e.g. dependent on the specific power drain).
The Qi standard is developed by the Wireless Power Consortium and more information can e.g. be found on their website: http://www.wirelesspowerconsortium.com/index.html, where in particular the defined Standards documents can be found.
In order to support the interworking and interoperability of power transmitters and power receivers, it is preferable that these devices can communicate with each other, i.e. it is desirable if communication between the power transmitter and power receiver is supported, and preferably if communication is supported in both directions. An example of a wireless power transfer system allowing communication between a power receiver and a power transmitter is provided in US2012/314745A1.
The Qi standard supports communication from the power receiver to the power transmitter thereby enabling the power receiver to provide information that may allow the power transmitter to adapt to the specific power receiver. In the current standard, a unidirectional communication link from the power receiver to the power transmitter has been defined and the approach is based on a philosophy of the power receiver being the controlling element. To prepare and control the power transfer between the power transmitter and the power receiver, the power receiver specifically communicates information to the power transmitter.
The unidirectional communication is achieved by the power receiver performing load modulation wherein a loading applied to the secondary receiver coil by the power receiver is varied to provide a modulation of the power signal. The resulting changes in the electrical characteristics (e.g. variations in the current draw) can be detected and decoded (demodulated) by the power transmitter.
However, a limitation of the Qi system is that it does not support communication from the power transmitter to the power receiver. Furthermore, load modulation such as developed for Qi may be suboptimal in some applications.
As an example, FIG. 1 illustrates a power supply path for typical induction heating appliance. The power provision comprises an AC/DC converter 101 which rectifies the input ac voltage (e.g. the mains). The rectified mains signal is fed to a DC/AC converter 103 (Inverter) which generates a high frequency drive signal which is fed to a resonant tank 105 (a tuned L-C circuit) and via this to a transmitter coil 107. The system includes a heating pan, which can be represented by a receiver coil 109 and a load R_Sole 111 (representing the Eddy current losses in the pan sole).
FIG. 2 illustrates the voltage waveforms of the power path of FIG. 1. The mains voltage Umains is rectified by the AC/DC converter 101 to the voltage Udc_abs. A large storage capacitor, which is used to buffer the rectified mains voltage, is normally not applied in these kinds of applications since it will add to the total mains harmonic distortion of this application. As a result, a varying DC voltage is generated by the AC/DC converter 101.
Because of the characteristics of the rectified voltage Udc_abs, the output voltage Uac_HF of the DC/AC converter 103 is shaped as shown in FIG. 2. The normal operating frequency of the inverter is in the order of 20 kHz to 100 kHz.
The transmitter coil 107, together with the receiver coil 109 and resistance R_sole, is basically the load of the DC/AC converter 103. However, due to the nature of this load (both inductive and resistive) a resonant circuit 105 is typically used in between the DC/AC converter 103 and this load in order to cancel the inductive part of the load. Furthermore, the resonant network 105 typically results in a reduction in the switching losses of the inverter typically used in the DC/AC converter 103.
Communication between receiver and transmitter in a system such as FIG. 1 is faced with multiple challenges and difficulties. In particular, there is typically a conflict between the requirements and characteristics of the power signal and the desires for the communication. Typically, the system requires close interaction between the power transfer and communication functions. For example, the system is designed based on the concept of only one signal being inductively coupled between the transmitter and the power receiver, namely the power signal itself. However, using the power signal itself for not only performing a power transfer but also for carrying information results in difficulties due to the varying nature of the power signal amplitude. For example, in order to modulate a signal on to the power signal, or to use load modulation, the power signal must have sufficient amplitude. However, this cannot be guaranteed for a power signal such as that of FIG. 2.
As a specific example, using a load modulation approach wherein the power receiver communicates data by load modulation (such as in the Qi system) requires that the normal load is relatively constant. However, this cannot be guaranteed in many applications.
E.g., if wireless power transfer is to be used to power a motor driven appliance (such as e.g. a blender), a power path similar to that of FIG. 1 can be used but with the load (corresponding to the heating pan) being replaced by a separate receive inductor (Rx coil), an AC/DC converter and the DC motor itself. Such a power path is illustrated in FIG. 3.
The typical voltage and current waveforms of such a wireless motor driven appliance are shown in FIG. 4. As illustrated, the motor current, Idc_motor, tends to be quite erratic and discontinuous. Near the zero crossings of the mains voltage, gaps appear in the motor current. This is caused by the rotation voltage of the motor. The DC/AC converter (Inverter) is only able to supply current to the motor if the voltage Uac_Rx is higher than the rotation voltage Udc_mot induced in the motor.
To control the speed (or torque) of the motor, a speed sensor (or current sensor) may be added to the system, together with a feedback loop from the speed sensor to the power transmitter. Because of the nature of the inverter (which could be a voltage or current source), the DC/AC converter (Inverter) is preferably incorporated in this feedback loop. Therefore communication is required between the appliance part (the power receiver) and the power transmitter part. This may be achieved by applying load modulation techniques at the appliance side, such that the load changes can be detected and demodulated at the transmitter side. This demodulated data can then include information of the motor speed (or torque), or indeed any other information that may e.g. be used to control the transmitter.
However, when a motor driven appliance draws current, the amplitude of this current is strongly related to the load of the motor. If the motor load is changing, the motor current is changing as well. This results in the amplitude of the inverter current also changing with the load. This load variation will interfere with the load modulation, resulting in degraded communication. Indeed, in practice it is typically very difficult to detect load modulation for loads that include a motor as part of the load. Therefore, in such scenarios, the number of communication errors is relatively high or the communication may utilize a very high data symbol energy, thereby reducing the possible data rate very substantially.
Hence, an improved power transfer system would be advantageous and in particular a system allowing improved communication support, increased reliability, increased flexibility, facilitated implementation, reduced sensitivity to load variations and/or improved performance would be advantageous.